stab in the back

stab in the back (2)  {n. phr.},  {slang}
An act or a lie that hurts a friend or trusting person; a promise not kept, especially to a friend.
John stabbed his own friend in the back by stealing from his store.
My friend stabbed me in the back by telling the teacher I was playing hooky when I was home sick.
Categories: noun slang